KEYWORDS:

SHORT DESCRIPTION:

• A suitable soil, with a good grain size distribution and a clay content of 10 to 25 %, can be compacted in a slightly moistened state to produce strong, dimensionally stable blocks.

• In order to increase their durability, a binder and/or waterproofing agent is added to the soil. Common binders are cement, lime and bitumen, and their proportions vary according to the quality of soil (see Earth, Soil, Laterite and Soil Stabilizers).

• The advantages of building with stabilized soil blocks compared with most other soil construction techniques are:

• higher compressive strength and greater water resistance;

• ability to carry away by hand immediately after production;

• small drying and storage space requirement, as the block can be stacked immediately or on the day after production;

• easy transportation of dried blocks with low breakage rate;

• possibility of building walls with a higher height to thickness ratio;

• savings in cost, material and energy, as no external rendering is needed on well stabilized compressed blocks;

• lower cost of production and energy input than for equivalent volume of burnt clay bricks or concrete blocks, which are alternatives to stabilized compressed soil blocks.

Soil Selection

• The most appropriate soils for stabilized block production have sand contents of about 75 %, and minimum clay content of 10 %. The shaded area in the chart gives the impression that very few soil types fall within this group, but in reality their availability is almost universal. It is excavated after removing 10 - 15 cm of the topsoil in order to exclude organic matter.

• To achieve satisfactory results, however, a series of field tests are essential. Wherever laboratory facilities are available, they should be made use of, as field tests are not sufficiently accurate.

FIGURE

Soil Preparation

• Soils are rarely found in the state required for block production. In most cases, they need to be ground and screened through a 5 mm wire mesh.

• Mixing should take place close to the block mould and all additives thoroughly blended in the dry state. Unlike mixing concrete, the predetermined quantity of water must be sprinkled for even distribution.

• Each mix must be checked by squeezing a lump in one hand and allowing it to drop on a hard surface from about 1 metre height. If the lump remains together, it is too moist; if it disintegrates completely, it is too dry. The correct moisture content will not moisten the hand, but will make a firm lump which breaks apart into several smaller pieces when dropped. When using cement as the binder, only so much material should be prepared, as can be used up in about 20 minutes.

Making the Blocks

• Compaction of the soil mix in a mould can be done dynamically (ie sudden impact by tamping) or statically (ie gradual compression). Static pressure is obtained by blockmaking machines, which has become the most common method.

• The simplest, but slowest and most tiring method of block production is by tamping the soil in a mould (usually with hinged or detachable parts).

• More efficiently, a block press is used, in which the soil mix is compressed to 60 %, or even 50 %, of its original volume. The machines are either manually operated or motorized, but the procedure always involves filling the mould(s), compacting the soil (sometimes after pre-compaction), demoulding the block and removal to the drying area (see ANNEX: Machines and Equipment). On average, a team of 3 people is generally needed to operate the machine and remove the blocks. They must be assisted by a team of 4 - 6 workers, who excavate and prepare the soil at the same pace as the blocks are produced.

FIGURE

Drying and Curing

• Unlike traditional unstabilized, hand-moulded mud blocks, which have to be left to dry where they are made, compressed soil blocks are carried to a shaded curing area. Weakly compacted blocks are laid in rows on the ground and stacked a day later, while denser blocks can be stacked up to 5 layers immediately.

• If bitumen stabilizer is used, drying can be completed within 5 days, whereas cement requires about 15 days and lime 25 days. With both cement and lime, the blocks must be kept moist for the first 5 days by daily sprinkling.

LOK BRIK System (Bibl. 22.04)

• This system, developed by Dr. A. Bruce Etherington of AIT, Bangkok, is a variation of standard compressed soil block constructions, by which the walls can be built with great accuracy and speed, even with unskilled labour.

• The interlocking soil-cement bricks are made in a modified CINVA-Ram brickmaking machine (see ANNEX), which has two parallel upward thrust pistons (to ensure more accurate dimensions of the finished block) and a system of positive and negative frogs (to form recesses or protruding parts).

• No mortar is needed for laying the bricks, but vertical holes are provided, into which grout (thin fluid mortar) is poured. Vertical steel reinforcement can also be inserted wherever necessary, making the construction earthquake resistant.

• Apart from costs saved in material and labour, the uniformity and accuracy of construction gives it an appealing finish, so that no rendering is needed and further costs are saved.

FIGURE